At present, most aircraft have one or more retractable landing gear units. Each landing gear, simply called a gear, is mounted in a housing of the fuselage called a “gear compartment”. In cruising phase, the gear compartment is closed by a set of doors in order to maintain the aerodynamic profile of the aircraft. In the landing or takeoff phase, the gear is extended i.e. it is outside its housing in a position known as the lowered position. Before the aircraft landing phase, the gear comes out of its housing, i.e. it goes from a retracted position to a lowered position, normally in an automatic way, at the pilot's request.
In what is called “normal” operation, the gear unit exits or gets extended automatically from the gear compartment. In the event of a malfunctioning of the system, an emergency system ensures that the landing gear is extended automatically.
In normal operation, the landing gear can only be extended when the doors of the gear compartment have been opened. An operating sequence for the gear release system is therefore planned, wherein the doors are first of all opened and then the landing gear is lowered. A reverse sequence of operation is used to retract the gear into the gear compartment after the aircraft takes off.
In emergency operation, the landing gear exits the gear compartment by gravity, causing the doors to open. The doors of the gear compartment include one or more main doors and one or more secondary doors that are independent of the main doors. The main door is designed to be closed once the gear is in the lowered position. The secondary doors are designed to let through the landing gear strut; they therefore remain open so long as the gear is extended.
When the aircraft is on the ground, the main door of the gear compartment is closed, the secondary doors remaining open so as to leave only a minimum space for the gear strut. However, it may be necessary to open the main door for maintenance.
At present, the gear extension system is a combined system, i.e. it uses several energy sources. More specifically, in normal operation, the system uses hydraulic energy associated with electrical energy. In emergency operation, the system uses electrical energy associated with a mechanical kinematic chain. During maintenance, the system uses human energy associated with mechanical energy.
For normal operation, the system comprises the following elements:                an electrically controlled valve or solenoid valve that connects the specific hydraulic circuit to the gear compartment with the general hydraulic circuit of the aircraft. This solenoid valve is controlled from the flight deck; it puts the hydraulic circuit of the gear compartment into operation.        a hydraulic and electrical control channel for the door: this system comprises: at least one door-locking catch that keeps the door in a closed position,                    at least one door actuator that provides for the mobility of the door, and            a door solenoid valve that controls the door actuator.                        
At present, the hydraulic and electrical door control channel is a hydraulic circuit called a door circuit. The door valve is the input point of this control channel. It is electrically controlled. When the valve is open, a fluid flows throughout the control channel, unlocking the door catch and feeding the door actuator, thus causing the door to open.
The system furthermore comprises a gear control channel comprising:                at least one gear-holding catch that maintains the gear in retracted position,        at least one gear actuator that provides for the mobility of the gear, and        a gear solenoid valve that controls the gear actuator.        
At present, the gear control channel is a hydraulic circuit called a gear circuit. The gear valve is the input point of this control channel. It is electrically controlled. When the valve is open, a fluid flows throughout the control channel, unlocking the gear-holding catch and feeding the gear actuator, thus causing the extension of the gear.
When the gear is in lowered position, the supply to the gear is cut off. The door actuator is then actuated to close the main door, the secondary doors remaining open so as to then leave only a minimum space for the passage of the gear strut. Once the main door is closed, the supply to the door actuator is cut off to prevent any untimely opening of said door. The hydraulic circuits specific to the gear compartment are then disconnected from the general hydraulic circuit.
The opening of the door and then the extension of the gear are obtained by means of a hydraulic circuit sequentially controlled by an electronic computer. Thus, the fact of having one solenoid valve for the doors and another solenoid valve for the gear makes it possible to open these valves at different instants, more or less spaced out in time, thus providing for a sequencing in the opening of the doors and the extension of the gear.
The sequencing of the raising of the gear into the gear compartment is the reverse of the sequencing of the gear extension. It comprises the same steps as those of the gear extension and is controlled electrically and hydraulically, but performed in reverse order.
Thus, in normal operation, the general control valve and the valves of the gear and door hydraulic circuit are electrically controlled from the flight deck, for example by the onboard computer for the aircraft landing gear, upon a command from the pilot.
In the event of a malfunctioning of the general hydraulic circuit or of the hydraulic circuit specific to the gear compartment, an emergency operation is planned. This emergency operation, known as gear free-fall is obtained by means of a specific system shown in FIG. 1. It shows the emergency system installed in the gear compartment.
FIG. 1 is a schematic view of a gear compartment in which a classic emergency system is: mounted. This system has an actuator 1 electrically controlled, for example, by the landing gear computer. This actuator 1 is mechanically connected by a kinematic chain 2 (also called a linkage) with the door catch 3, the gear catch 7 and a valve 4. This figure shows a more detailed view of the door control channel 6 with its locking hook 3 and the gear control channel 5 with its holding catch 7. The valve 4 enables the pistons to slide freely in the actuators for the door and the gear.
In this emergency operation, the sequencing of the door and gear catch unlocking operations is done mechanically by the implementation of the actuator 1.
It will be understood of course that in this mode of emergency operation, the doors remain open and, the landing gear cannot be retracted.
In another mode of operation of the gear extension system, especially when the aircraft is on the ground it is important for reasons of maintenance to be able to open the main door of the gear compartment manually. For, when the aircraft is on the ground and when it is resting on its landing gear, this door is closed. The maintenance staff should be able to enter the landing gear compartment for inspection. On the ground, the hydraulic energy is not available because the engines are turned off. The door is then opened mechanically using a mechanical grip situated close to the door. FIG. 2 shows one example of a classic maintenance system of this kind. It provides a schematic view of the maintenance system as fixed in the landing gear compartment.
The maintenance system comprises a mechanical grip 8 connected by a kinematic chain 9 (also called a linkage) to the door control channel 6. The kinematic chain 9 may be a ball control unit fixed by guides to the wall of the landing gear compartment. The actuation of the grip 8 causes the kinematic chain 9 to be driven in motion, actuating the opening of the main door. More specifically, when the grip 8 is rotated, example by an angle of 90°, the kinematic chain 9 is driven in a linear motion which achieves the following sequencing:                the isolation of the specific hydraulic circuit for supplying the door actuators (for the sake of safety, in order to prevent an accidental connection of the general hydraulic circuit from causing an untimely closing of the main door);        the opening of the valve 10 enabling the free circulation of fluid in the chamber of the door actuator so that the actuator does not counter the opening of said door;        the mechanical unlocking of the main door catch.        
The main door then opens by gravity or by a manual operation.
In the maintenance system, the manual grip is fixed to the fuselage of the aircraft, generally in a closed cavity, made in the vicinity of the door. The consequence of this is that it is placed at a relatively great height from the ground. Consequently, the maintenance staff in certain cases have to climb a ladder to be able to grasp this grip and actuate it in order to open the door.
Moreover, as just explained, the architecture for controlling the extension and retraction of the landing gear is complex and requires a large number of bulky elements, such as the kinematic chains which differ according to the mode of operation.
Furthermore, so that they may capable of being used in the different modes of operation, certain elements are controllable by different energy sources and this makes them more complex. For example, the gear catch has two independent inputs: one hydraulic input for normal operation and one mechanical input for emergency operation. Similarly, the door catch has three separate, independent inputs: one hydraulic input in normal operation, one mechanical input in emergency mode and one special mechanical input for maintenance. This leads to substantial requirements of space for the interfaces with the control channels.
Furthermore, the door and gear catches are situated in non-pressurized compartments while the control channels are situated mainly in pressurized compartments. Consequently, all manner of precautions must be taken to enable this linkage to go through the walls of the pressurized compartments without affecting the pressure in these compartments.
At present, aircraft manufacturers are seeking to design aircraft to transport ever bulkier and heavier loads in a single flight. To this end, they are seeking to design ever larger aircraft. Such aircraft may have a greater number of landing gear than classic aircraft. This greatly increases the above-stated problems for landing gear. Furthermore, this increase in the number of landing gear units considerably increases the mass of the aircraft.
Furthermore, in these large-sized aircraft, the wheels of the landing gear may be bulkier than the wheels of classic landing gear. This means that the fuselage is at a higher position relative to ground. For reasons of mass and positioning, the grip must be placed in a non-pressurized zone which requires that an orifice be made for the passage of the linkage into a pressurized wall with all the pressurization-related problems that this may entail. This grip would then be positioned at a great distance from the ground, at a height greater than man-size height, with increased risks of falling for maintenance crew.